1. Field of the Invention
This invention pertains generally to a quasi-static fiber pressure sensor and mor articularly to a quasi-static fiber pressure sensor using self-referenced fiber spectral interferometry.
2. Description of the Related Art
Precise, real time, remote, self-calibrated measurement of quasi-static pressure is of fundamental significance to the sensing community. Remote optical measurement of pressure-induced plate deflection is often obtained via white light interferometry or dual wavelength illumination, which require a path matching demodulator for coherent addition of the beams reflected from the two sides of the gap. Those techniques suffer from several drawbacks; first, since the system infers the value of the pressure from the difference in gaps between the sensor cavity and the demodulator, one needs to keep the demodulator length free from any drift due to environmental perturbations within the resolution of the instrument. Secondly, the two paths must be either precisely matched to obtain coherent addition, or require a fine precision linear translation stage. Since white light sources do not efficiently couple into optical fibers, one typically uses LEDs or multi-mode lasers.
The problems associated with implementing dual wavelength demodulation are; first, one needs to select two sources with a predetermined wavelength difference that will cover the range of deflections and yield an unambiguous solution. In order to maintain the proper accuracy on pressure, the sources mean wavelengths must be stabilized, requiring active current and temperature laser controllers. Secondly, one needs to ensure that the coherence function of both sources is matched so that only the term corresponding to the difference in cavity lengths contributes to a phase term. Otherwise interference between the other path delayed beams will show as spurious modulation signals. This implies that the spectra of the sources cannot vary with age or feedback, within the required accuracy.